1. Field of the Invention
The present invention relates to a light-receiving method of an avalanche photodiode (APD). and a bias control circuit for the APD.
2. Related Prior Art
The APD, which uses a physical phenomenon of the avalanche breakdown of the semiconductor p-n junction at a high reverse bias voltage, has a multiplication factor greater than unity. The multiplication factor means that how many electrical carriers can be generated by a signal photon. Therefore, the APD can generate a large photo current from a weak optical signal. The design or the specification of the circuit connected to the APD strongly depends on how large the multiplication factor thereof is set. The PIN-PD, which is a semiconductor light-receiving device similar to the APD, generally has a multiplication factor smaller than unity because the PIN-PD shows no avalanche breakdown phenomenon.
When a large multiplication factor is set by applying the high reverse bias voltage to the APD, a noise involved in the photo current output from the APD will also increase. On the contrary, a small multiplication factor leads the optical sensitivity of the APD insufficient to a presetting specification. The Japanese patent application published as H09-321710 has disclosed that the bias voltage to the APD is controlled to maximize the signal-to-noise ratio (SNR) thereof. The method uses two filters, one of which extracts the signal component and the other extracts the noise component. The SNR is calculated from thus extracted signal and noise components, and the bias voltage is applied to the APD so as to maximize the calculated SNR,
Japanese patent application published as 2000-244418 has disclosed a method for controlling the bias voltage to the APD based on an optical input level and ambient temperatures. In this disclosure, a PIN-PD provided in addition to the APD receives the input light, and the bias voltage applied to the APD is controlled by the reference signal generated by the PIN-PD. This patent application has also disclosed that the bias voltage applied to the APD is adjusted based on the ambient temperature.
These prior applications have disclosed that the bias voltage applied to the APD, namely the multiplication factor thereof, may be adjusted as the change of the optical input level of the temperature. However, as shown in FIG. 7, the photo current output from the APD changes even under the constant bias voltage. Especially, in a region of the bias voltage from 30V to 60V where a significant multiplication factor is obtained, the output photo current, i.e. the multiplication factor, widely changes as the temperature varies.
The PIN-PD has a quite smaller temperature dependence of the multiplication factor compared to that of the APD, and the magnitude thereof is nearly unity. Accordingly, by using the PIN-PD as a monitor device for the input light and controlling the bias voltage applied to the APD based on the output from the PIN-PD, the multiplication factor of the APD, especially a drift for the temperature, may be suppressed. However, to prepare the PIN-PD independently on the APD becomes an apparatus to be complex, and to adjust the light-receiving condition between the APD and thus prepared PIN-PD may be a troublesome procedure. Thus, the independent PIN-PD on the APD may not appropriately control the bias voltage to the APD.